Estrogens refer to a group of endogenous and synthetic hormones that are important for and used for tissue and bone maintenance. Estrogens are endocrine regulators in the cellular processes involved in the development and maintenance of the reproductive system. The role of estrogens in reproductive biology, the prevention of postmenopausal hot flashes, and the prevention of postmenopausal osteoporosis are well established. Estradiol is the principal endogenous human estrogen, and is found in both women and men.
The biological actions of estrogens and antiestrogens are manifest through two distinct intracellular receptors, estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). Endogenous estrogens are typically potent activators of both receptor subtypes. For example estradiol acts as an ERα agonist in many tissues, including breast, bone, cardiovascular and central nervous system tissues. Selective estrogen receptor modulators commonly act differently in different tissues. For example, a SERM may be an ERα antagonist in the breast, but may be a partial ERα agonist in the uterus, bone and cardiovascular systems. Compounds that act as estrogen receptor ligands are, therefore, useful in treating a variety of conditions and disorders.
Prostate cancer is one of the most frequently diagnosed noncutaneous cancers among men in the US and is the second most common cause of cancer deaths with over 180,000 new cases and almost 29,000 deaths expected this year. Patients with advanced prostate cancer undergo androgen deprivation therapy (ADT), typically either by luteinizing hormone releasing hormone (LHRH) agonists or by bilateral orchiectomy. Androgen deprivation therapy not only reduces testosterone, but estrogen levels are also lower since estrogen is derived from the aromatization of testosterone, which levels are depleted by ADT. Androgen deprivation therapy-induced estrogen deficiency causes significant side effects which include hot flushes, gynecomastia and mastalgia, bone loss, decreases in bone quality and strength, osteoporosis and life-threatening fractures, adverse lipid changes and higher cardiovascular disease and myocardial infarction, and depression and other mood changes. It is believed that many of the estrogen deficiency side effects of ADT are mediated by ERα.
Leuprolide acetate (Lupron®) is a synthetic nonapeptide analog of naturally occurring gonadotropin-releasing hormone (GnRH or LH-RH). Leuprolide acetate is an LH-RH superagonist that eventually suppresses LH secretion by the pituitary. Leuprolide acetate acts as a potent inhibitor of gonadotropin secretion, resulting in suppression of ovarian and testicular steroidogenesis. In humans, administration of leuprolide acetate results in an initial increase in circulating levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH), leading to a transient increase in levels of the gonadal steroids (testosterone and dihydrotestosterone in males, and estrone and estradiol in premenopausal females). However, continuous administration of leuprolide acetate results in decreased levels of LH and FSH. In males, testosterone is reduced to castrate levels (below 50 ng/dL). In premenopausal females, estrogens are reduced to postmenopausal levels. Testosterone is a known stimulus for cancerous cells of the prostate. Suppressing testosterone secretion or inhibiting the actions of testosterone is thus a necessary component of prostate cancer therapy. Leuprolide acetate can be used for LH suppression, which is the reduction and lowering of serum testosterone to castrate levels to treat prostate cancer.
Prior to the introduction of LHRH agonists, castrate testosterone levels were achieved by increasing estrogen activity in the pituitary via estrogens, primarily diethylstilbestrol (DES). DES was equally effective as LHRH agonists at suppressing testosterone to castrate levels. Patients treated with DES did not have hot flashes or bone loss, but did have gynecomastia at higher rates than ADT with LHRH agonists. Unfortunately, highly potent, pure estrogens, like DES and estradiol, are often associated with a high risk of severe cardiovascular and thromboembolic complications which have limited their clinical use.
The compounds of this invention suppress testosterone levels to castrate levels which may be used to treat prostate cancer, while preventing the increased risk of thrombotic events, and without causing bone loss, hot flashes and/or gynecomastia.